Sheet material and method of making



V Patented M916, 1930 AL ERT E. H.1AIR, or'BELMoNT,

MASSACHUSETTS, ASSIGNOR oFncE To ARTHUR D. LITTLE,

INCORPORATED, OF CAMBRIDGE, MASSACHUSETTS,. A CORPORATION OF MASSA- GEUSETTS .ll'o Drawing.

This invention relates to sheet materials having improved dielectric properties, adapted for use in condensers and the like, and provides' a method of making the same.

In the art of making electrical condensers it has heretofore been regarded as essential to employ a paper made from rag fibres, which could be saturated with a suitable dielectric material suchas wax or a cellulosic sheet impervious to oils and waxes and possessing in itself adequate dielectric strength. Rag or bast fibre, however, is relatively expensive for such use and other materials which have been tried either do not possess suificiently good dielectric properties or other electrical characteristics and are therefore unsuitable.

Accordingly, it is an object of this invention to provide a method of making a condenser sheet which shall make use of inexpensive materials and yet produce a product of high dielectric strength. It is also an object of the invention to obtain a sheet which shall possess these properties and be relatively thin. Other objects will appear from the followin disclosure.

It is foun as a part of this invention, that relatively pure alpha cellulose fibres (such as those obtamed from coniferous woods by well i known methods of treatment) are intrinsicala high dielectric strength. fibres felt together under for manufacture into palFy characterized b urthermore, suc the usual treatment per to produce a web or mat which possessesa marked degree of absorptive capacity. In the course of fabricating such fibres into paper, however, it is difiicult to develop appreciable strength except in relatively thick ayers or sheets while condenser sheets, as.

above mentioned, are preferably thin.

On the other hand, it is found that fibres such as those of a sulphate or kraft pul -(which yield a stronger sheet of paper an may, accordingly be made into much thinner sheets than alp a cellulose pulp), are less pervious or -impervious to impregnating ma terials and" do not themselves possess so good -dielectric properties. Consequently, while they may be made into thin sheets such product are not per se wholly satisfactory and are not adapted to receive an impregnation SHEET MATERIAL AND METHOD or MAKING Application filed July 1, 1929. Serial No. 375,344.

of dielectric material to improve them in this respect. A r

The method of the present invention includes the preliminary preparation of a high alpha cellulose; pul and of a sulphate or kraft pulp, followe by mixing the two pulps in suitable pro ortrons, running the mixed pulp into the oi'm of a sheet or web, and passing through'rolls or calenders, in the usual manner, to produce a paper sheet of the desired thickness and density, in accordance with aper-making practices. The sheet produced may then be lmpregnated or saturated with a suitable dielectric substance in the customary manner.

The preparation of the alpha cellulose includesa mild but prolonged beating of the pulp (e. g., a pulp of over 93% alpha cellulose and approximately 6% concentration) to reducethe average size of the fibres, preferably with the rolls so'set as to avoid cutting action on the fibres and without inducing appreciable hydration.

The sulphate or also have a concentration of 6%, for example, and is preferably freshly prepared) is more resistant than the alpha cellulose and may be subjected to a tion. This tends to brush out or fray the fibres and at the same time reduces them preferably without cutting action) to finer iorm. The ultimate fibres, are nevertheless,

stronger and tougher than the alpha cellulose fibers and while they may include a few shives the ro ortionate amount or number is reduced t e beating treatment. At the same time, tile fibre undergoes an appreciable hydration, although the alpha cellulose content, upon determination is frequentlyjound to be in the vicinity of 80 to 87%.

The beating treatment of both pulps is controlled by repeated examinations of the fibre under the microscope. The rolls are set wider apart, if necessary, to avoid any observed tendency to cut across the fibres, or closer toether if the beating action is too slow. The mdication in the alpha cellulose of a suflicient kraft pulp (which may more drastic beating acreduction in fibre slze or fibration' determines a the completion of the treatment. In the knit pulp the same criteria are observed as well as .the degree of hydration of the pulp which time is required in both cases to yield a satisfactorily beaten pulp, this period being usually in excess of twenty-four hours;

The furnish is then prepared from these two pulps by mlxing them in appropriate J quantities. The larger the proportlon .of

high alpha cellulose pulp employed, the more open' or porous the resulting paper will be. But it is not practicable to use an all alpha cellulose wood pulp since it does not produce a paper sufficiently strong to resist necessary handling. Moreover, since the alpha cellulose tends to make a more open sheet the resulting paper will absorb a high proportion of impregnant.

On the other hand, a paper sheet made entirely' from kraft pulp, while strong, has poorer dielectric properties and is non-absorbent of impregnating material. I It may also contain moisture, which is enclosed during manufacture and is not removable by dryin and it is tlfought that such moisture may ecrease its value as .a dielectric when used alone. I

By the addition of kraft pulp to alpha cellulose pulp, however, (in amounts up vto 60%) it is found that the strength of the finished sheet is greatly increased .without destroying the inherent properties of thealpha cellulose, and the resulting sheet may be more readily formed much thinner and without decreasing its dielectric pro erties.

For example 35% to 45% of kra t pulp may be added to 65 to 55% alpha pulp, the

tmixture being uniform and then run on a Fourdrinier machine in the usual way to produce a-sheet of paper of the required thickness. 1 1 v In the usual practice of making condensers, the aper sheet thus roduced, .in one or more t icknesses as may LEE required, with alternate sheets of metal foil, is rolled into cylindrical form upon a core. After removing the core, this cylinder is placed in. a'

7 closed chamber which is evacuated and heated to a temperature of about 200 F;, and held under these conditions for a number of hours to efiectively dry the whole roll. Melted wax or paraflin (having a melting point below the temperature of the. chamber, for example) is now admitted to the chamber and, (when the v chamber is filled) subjected to' pressure. The wax is finally allowedto drain off; the wax impregnated roll is removed, and the impregnated roll is subjected to hydraulic pressure which flattens it into the customary, oval shape in which these condenser units "are generally used. r

The finished condenser thus obtained is characterized by improved dielectric properties, and also possesses uniformity of density and impregnation.

I claim: 1. Method of making dielectric sheet material 'for condensers and the like,-comprising as steps preparing a high alpha cellulose wood pulp and a sulphate pulp, mixing said 3. Method of makmg dielectric sheet ma- I terial for condensers and the like, comprising as steps preparing an alpha cellulose pulp and a sulphate pulp, mixing the pulps, forming the resulting mixture into a'sheet of. paper, and impregnating with molten wax.

4. Method of making dielectric sheet material for condensers and the like, comprising as steps preparing an, alpha cellulose pulp and a sulphate. pulp, mixing the pulps, forming the resulting mixture into a sheet of paper, drying and evacuating the same and finally .'mpregnating with a molten dielectric substance.

'5. Method of making dielectric sheet material for con ensers, comprising as steps preparing an alpha cellulose pulp and a sulphate pu sub ecting thesame to a beating operation, mixing the ulps in the proportion of 10% 'to 60% of the sulphate pulp, and forming the mixed ulp into a sheet of paper.

6. Method of ma 'ng dielectric sheet material for condensers, comprising as stepspreparing an alpha cellulose pulp and a sulphate pulp, sub ecting the same to a heating operation, mixing the pulpsin the proportion of 35% to 45% of the sulphate pulp, and forming the mixed pul into a sheet of paper.

7. A sheet material or condensers and the like, comprising alpha cellulose fibres Bonded together with sulphate fibres.

the like, comprising alpha cellulose fibres...

bonded together with sulphate fibres, and characterized by being absorbent and by having hi h dielectric properties. i Y

9. S eet'material for condensers and the like comprising alpha cellulose fibre and 10% to 60% of sulphate fibre, in uniform distribution and admixture.

10. Sheet material for condensers and the i like comprising alpha cellulose fibre and 35% to 45% of sulphate fibre, in uniform distribution and admixture.

11. Sheet material for condensers and the like comprising alpha cellulose fibre and 10% to 50%.of sulphate fibre, in uniform distribution and admixture and impregnated with a solid dielectric.

12. Sheet material for condensers and the like comprising alpha cellulose fibre and 35% to45% of sulphate fibre, inuniform distribution and admixture and impregnated with. a solid dielectric.

13. A ulp for the manufacture of sheet material or condensers and the like comprising 10% to 60% of a sulphate pulp, and a pulp of high al ha cellulose content.

14;. A ulp or the manufacture of sheet material or condensers and the like comprising 35% to 45% of a sulphate pulp, and a pulp of high al ha cellulose content.

Signed y me at Boston, Massachusetts, this 28th day of J une, 1929. j a

' ALBERT E. H. FAIR. 

